1. Field of the Invention
The present invention relates to a color cathode-ray tube (hereinafter referred to as "color CRT") capable of allowing optimum beam alignment and, more specifically, to a color CRT having an explosion-proof band shrink fitted around the periphery of the panel thereof so as to brace the panel appropriately according to an amount of correction necessary for proper alignment of electron beams. Furthermore, recesses are formed in the explosion-proof band to control the effective sectional area of the explosion-proof band according to an amount of correction to be made for aligning the electron beams.
2. Description of the Prior Art
In a conventional color CRT, as illustrated in FIGS. 10 to 12, an explosion-proof band 3 is shrink fitted around the periphery of the panel 2 of a tube body 1 to reinforce the tube body 1. FIGS. 10 and 11 illustrate CRTs each having a cylindrical panel 2, while FIG. 12 illustrates a CRT having a spherical panel 2. As shown in FIGS. 10 to 12, the lugs 4 are integrally attached to the corners of the explosion-proof band 3 for mounting the CRT on a frame.
When the tube body 1 is evacuated to a high vacuum, the panel surface and the general configuration of the tube body are deformed as illustrated in FIG. 14 and a large stress concentrates in the peripheral portion of the panel. Accordingly, the tube body is reinforced by the explosion-proof band 3, principally to apply an external force to the peripheral portion of the panel so that the stress is minimized and the original shape of the panel surface is restored to the maximum extent possible as indicated by broken lines. The band is called an explosion-proof band because it resists outward (or exploding) motion of the periphery of the tube. Thus, since the principal purpose of providing the explosion-proof band is to prevent the explosion of the tube body, it has been a conventional practice to control the recovery .delta. of the strain to reduce the strain to a minimum value and hence the variation of the recovery .delta.. For example, in a 20-inch class CRT, .delta. has been in the range of .+-.150 .mu.m.
In the industrial high-precision fine color CRT, as compared with the TV use color CRT, there is a small electron beam alignment tolerance for the fluorescent screen, for example, on the fluorescent stripes. In a color CRT, misalignment is liable to occur in areas A and B on both sides of the central area of the panel 2, as viewed from the front side of the panel 2, as illustrated in FIG. 13. In the areas A and B, the panel glass is subject to deformation (concave deformation) when the tube body is evacuated, and the positional variation of the fluorescent stripes is likely to occur when the conditions of the fluorescent screen forming process are not appropriate. Consequently, misalignment of electron beams occurs in the finished CRT, and hence the color purity of such a CRT is unsatisfactory.
On the other hand, as described hereinbefore, the tube body is reinforced by the explosion-proof band 3. However, the variation in the recovery .delta. of strain directly influences the color purity of the CRT. It has been a usual practice to correct misalignment in the areas A and B by adjusting the correction lens system in the fluorescent surface forming process. This conventional method is able to correct the apparent recovery .delta. of strain of every lot of CRTs, however, the method is unable to correct the recovery .delta. of strain of every CRT in a lot.